Music controllers are interface devices that are operated by a musician to produce musical sound in a similar way to performing on an acoustic musical instrument. The music controller generates signal data that can be used to control an electronic sound generating device. The music controller and the sound generating device together function as an electronic musical instrument. The sound generating device may emulate an acoustic musical instrument or produce an entirely synthetic sound, and can use a variety of synthesis methods including sample-playback and physical modelling. The most common types of music controller are keyboard devices which operate like a conventional piano keyboard but other types can be used to simulate bowed instruments. It is known for music controllers in the form of bowing sensors to use optical devices to track bow movement. Such music controllers require a specially prepared bow, either with finely spaced marks or a precise graduation of transparency. It is desirable therefore to find a simpler and more practical method of tracking bow movement.